Lithographic printing with polarized light

ABSTRACT

The present invention provides systems and methods for improved lithographic printing with polarized light. In embodiments of the present invention, polarized light (radially or tangentially polarized) is used to illuminate a phase-shift mask (PSM) and produce an exposure beam. A negative photoresist layer is then exposed by light in the exposure beam. A chromeless PSM can be used. In further embodiments of the present invention, radially polarized light is used to illuminate a mask and produce an exposure beam. A positive photoresist layer is then exposed by light in the exposure beam. The mask can be an attenuating PSM or binary mask. A very high image quality is obtained even when printing contact holes at various pitches in low k applications.

CLAIM TO PRIORITY

[0001] This application claims priority to U.S. Provisional Appl. No.60/448,530, filed Feb. 21, 2003, incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to high numerical aperture andimmersion lithography.

[0004] 2. Related Art

[0005] Lithographic tools and techniques are increasingly called upon toprint patterns at a high resolution. For example, in the manufacture ofsemiconductor dies or chips, patterns of circuit features, such aslines, contact holes, or other elements, often need to be printed at ahigh resolution to improve the packing density of circuit elements andreduce the pitch of the pattern. Certain circuit features, such ascontact holes or vias, are especially difficult to fabricate.

[0006] A well-known parameter relating to lithography resolution is thecritical dimension (CD). The CD is the size of the smallest geometricalfeatures which can be formed during semiconductor device and circuitmanufacturing using a given technology. The critical dimension can bedescribed as shown in the following function:

CD=k (λ/NA),

[0007] where λ is a wavelength used in lithography, NA is the numericaperture, and k is the dielectric constant. Among the trends inlithography is to reduce the CD by lowering the wavelength used,increasing the numeric aperture, and reducing the value k.

[0008] Printing can be difficult in low k applications. For example,contact holes are difficult to print when k is less than 0.5. Ahigh-contrast image of sufficient quality that includes groups ofcontacts holes like contact arrays is especially hard to print.

[0009] Techniques to enhance contrast using a very high NA and off-axisillumination have been used but these techniques fail for small pitches.For example, at 157 nm wavelength, 0.93 NA, the limiting pitch (based onresolution) is roughly 135 nm (k=0.4)—which is too high for certainapplications. Also, a forbidden pitch may occur. This means that if theillumination is optimized for a given pitch, printing other pitchessimultaneously may become impossible. Forbidden pitch can be manifestedin a low normalized image log slope (NILS) or poor CD control for theforbidden pitch.

SUMMARY OF THE INVENTION

[0010] The present invention provides systems and methods for improvedlithographic printing with polarized light.

[0011] In embodiments of the present invention, polarized light (forexample, radially, tangentially, or custom polarized) is used toilluminate a phase-shift mask (PSM) and produce an exposure beam. Anegative photoresist layer is then exposed by light in the exposurebeam. A chromeless PSM can be used. In one example embodiment, radiallypolarized light is used in conjunction with chromeless PSMs, Cartesianquadrupole (C-quad) illumination and negative photoresists. A very highimage quality is obtained even when printing contact holes at variouspitches in low k applications. Forbidden pitch is avoided.

[0012] In further embodiments of the present invention, radiallypolarized light is used to illuminate a mask and produce an exposurebeam. A positive photoresist layer is then exposed by light in theexposure beam. The mask can be an attenuating PSM or binary mask. In oneexample embodiment, radially polarized light is used in conjunction withattenuating phase-shift masks or binary masks, standard diagonalquadrupole illumination and positive photoresists. A very high imagequality is obtained even when printing contact holes at various pitchesin low k applications.

[0013] To further improve printing, a custom polarization can be used.The custom polarization may be, for example, a combination of radial andtangential polarization. In addition, an alternating PSM can also beused to improve print quality.

[0014] Further embodiments, features, and advantages of the presentinventions, as well as the structure and operation of the variousembodiments of the present invention, are described in detail below withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0015] The accompanying drawings, which are incorporated herein and forma part of the specification, illustrate the present invention and,together with the description, further serve to explain the principlesof the invention and to enable a person skilled in the pertinent art tomake and use the invention.

[0016]FIG. 1 is a lithography system according to an embodiment of thepresent invention.

[0017]FIG. 2 is a lithography system according to an embodiment of thepresent invention.

[0018]FIG. 3A is a zoomed-in image of contact holes in resist on awafer.

[0019]FIG. 3B is a top-view image of contact holes in resist on a wafer.

[0020]FIGS. 4A and 4B illustrate 2D attenuating PSM mask spectra,p_(x)=p_(y)=p with on-axis and off-axis illumination respectivelyaccording to an embodiment of the present invention.

[0021]FIG. 5 is an image that shows a limiting pitch with unpolarizedlight, where C₀ and C₄₅ are the orthogonal and diagonal contrasts,respectively.

[0022]FIG. 6 is an illustration of an example simulation experiment(radially polarized light is shown for illustration).

[0023]FIGS. 7A and 7B show effects of radial and tangential polarizationon image quality.

[0024]FIGS. 8A-8C show a comparison of three polarization modes in anexample of 125-nm pitch (45-degree-rotated masks).

[0025]FIGS. 9A-9C illustrate effects of polarization on image quality.FIG. 9A illustrates a poor contrast image of grouped contact holesobtained in a case using unpolarized light. FIG. 9B illustrates a poorcontrast image of grouped contact holes obtained in a case usingtangentially polarized light. FIG. 9C illustrates a high contrast imageof grouped contact holes obtained in a case using radially polarizedlight according to an embodiment of the present invention.

[0026]FIG. 10A illustrates the effect of a tangential polarizer onelectric field vectors in light.

[0027]FIG. 10B illustrates the effect of a radial polarizer on electricfield vectors in light.

[0028]FIGS. 11A and 11B are diagrams that show through pitch behaviorusing radially polarized light with a chromeless, alternating PSMaccording to an embodiment of the present invention.

[0029]FIG. 12 is an illustration of an attenuating PSM.

[0030]FIG. 13 is an illustration of a binary PSM.

[0031]FIGS. 14A-14C are images that show effects of polarization onimage quality in the case of an attenuating PSM, 125 nm pitch accordingto an embodiment of the present invention.

[0032]FIG. 15 is an illustration of an alternating PSM.

[0033]FIG. 16 shows a chromeless alternating PSM mask layout.

[0034]FIGS. 17A and 17B show diffraction patterns for the 2D chromelessalternating PSM for on-axis and off-axis illumination respectively. FIG.5 is an image that shows a limiting pitch with unpolarized light, whereCo and C₄₅ are the orthogonal and diagonal contrasts, respectively.

[0035]FIGS. 18A and 18B show images in air (a) and in resist (b) usingan example chromeless alternating PSM.

[0036]FIG. 19 shows six aerial images at best focus vs. pitch usingchromeless alternating PSMs and radially polarized light with C-quad.

[0037]FIGS. 20A and 20B are graphs that show aerial imagecharacteristics versus pitch.

[0038]FIGS. 21A and 21B are example custom polarization maps.

[0039]FIG. 22 is an immersion image in the case of unpolarized lightwith diagonal quadrupole and 6% attenuating PSM at n=1.5.

[0040]FIG. 23 is an aerial image at best focus under extreme ultravioletradiation (EUV) conditions.

[0041] The present invention will be described with reference to theaccompanying drawings. The drawing in which an element first appears istypically indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION OF THE INVENTION

[0042] Table of Contents

[0043] 1. Overall System

[0044] 2. Discussion and Simulation Results

[0045] A. Introduction

[0046] B. Resolution

[0047] B.1. Theoretical resolution limits

[0048] B.2. Resolution capability with off-axis illumination lithography

[0049] C Polarization

[0050] C.1. Simulation Experiment

[0051] C.2. Effect of Polarization on Image Quality

[0052] C.3. Polarized light, Chromeless PSM, Negative photo-resist

[0053] C.4. Radially polarized light, attenuating phase-shift masks orbinary masks, and positive photoresists

[0054] D. Polarization With Chromeless Alternating PSM

[0055] D.1. Chromeless Alternating PSM In Conjunction With RadiallyPolarized Light, 100-nm Pitch Nested Contacts

[0056] D.2. Through-Pitch Behavior, Chromeless Contacts With RadiallyPolarized Light

[0057] D.3. Custom Polarization

[0058] E. Immersion Lithography

[0059] F. EUV

[0060] While specific configurations and arrangements are discussed, itshould be understood that this is done for illustrative purposes only. Aperson skilled in the pertinent art will recognize that otherconfigurations and arrangements can be used without departing from thespirit and scope of the present invention. It will be apparent to aperson skilled in the pertinent art that this invention can also beemployed in a variety of other applications.

[0061] The present invention provides systems and methods for improvedlithographic printing with polarized light.

[0062] 1. Overall System

[0063]FIG. 1 is a lithography system 100 according to an embodiment ofthe present invention. System 100 includes an illumination source 102.In an embodiment, illumination source 102 emits pre-polarizedillumination light along an optical path. Although the present inventionis described herein with reference to pre-polarized illumination light,one of skill in the art will recognize that unpolarized illuminationlight may also be used. For example of pre-polarized light, illuminationsource 102 can be a laser that emits a laser beam with a tendency to beapproximately linearly polarized. Alternatively, a polarizer could beadded within a laser generator in polarized illumination source 102.

[0064] The pre-polarized light then passes through a pattern polarizingdevice 104. As used herein, pattern polarizing device 104 is defined toencompass any polarizing device including, but not limited to,traditional and custom polarizers and wave plates. If pre-polarizedlight is emitted by illumination source 102, pattern polarizing device104 may be any polarizing device such as, for example, one or more of apolarizer or a wave plate. If unpolarized light is emitted byillumination source 102, pattern polarizing device 104 is a polarizerrather than a wave plate.

[0065] Pattern polarizing device 104 shapes the pre-polarizedillumination light into various predetermined arrangements, such aspolarization patterns and intensity patterns. For example, patternpolarizing device 104 may shape the pre-polarized illumination lightinto radially polarized light, tangentially polarized light, or lightwith a custom polarization. In an embodiment, the illumination light isquadrupole illumination, such as Cartesian quadrupole (C-quad)illumination. Although quadrupole illumination is used herein as anexample, one of skill in the art will recognize that illumination of anysource shape may be used.

[0066] The illumination light illuminates mask 106. Mask 106 produces adesign in the illumination light. One of skill in the art will recognizethat mask 106 can be any type of mask or reticle. In an embodiment ofthe present invention, mask 106 is a binary mask. In other embodiments,mask 106 is a phase-shift mask (PSM), such as, for example, a chromelessPSM, alternating PSM, or attenuating PSM.

[0067] The light including the mask design then passes throughprojection optic 108, which further conditions and processes the light.Projection optic 108 may include one element or a plurality of opticalelements. Projection optic 108 produces an exposure beam that continuesalong the optical path.

[0068] Finally, the exposure beam exposes wafer 110 according to thedesign carried by the exposure beam. In embodiments of the presentinvention, wafer 110 is covered by a negative photoresist layer. In oneexample embodiment, radially polarized light is used in conjunction withchromeless PSMs, C-quad illumination and negative photoresists. A veryhigh image quality is obtained even when printing contact holes atvarious pitches in low k applications. Forbidden pitch is avoided.

[0069] In other embodiments of the present invention, wafer 110 iscovered by a positive photoresist layer. For example, radially polarizedlight is used in an embodiment to illuminate mask 106 and produce anexposure beam. A positive photoresist layer is then exposed by light inthe exposure beam. In another embodiment, radially polarized light isused in conjunction with attenuating phase-shift masks or binary masks,standard diagonal quadrupole illumination and positive photoresists. Avery high image quality is obtained even when printing contact holes atvarious pitches in low k applications.

[0070]FIG. 2 is another example lithography system 200 in which thepresent invention can be implemented. Polarized illumination source 102and mask 106 perform the same functions as described with respect tosystem 100. In system 200, however, a pattern polarizing device 202 isincluded in projection optic. As with pattern polarizing device 104,pattern polarizing device 202 shapes the pre-polarized illuminationlight into various predetermined arrangements, such as a radialpolarization arrangement, a tangential polarization arrangement, or acustom polarization arrangement. Optical components for further shapingand conditioning the illumination light may be placed before and/orafter pattern polarizing device 202. These optical components are shownas projection optic 204A and projection optic 204B, and create anexposure beam that continues along the optical path.

[0071] After being shaped by pattern polarizing device 202 andprojection optics 204A and/or 204B, the polarized light exposes wafer110 according to the design produced by mask 106. As stated with respectto FIG. 1, wafer 110 may be covered with either a positive or negativephotoresist layer.

[0072] 2. Discussion and Simulation Results

[0073] The following discussion and simulation results is provided tofurther illustrate aspects and features of embodiments of the presentinvention, and is not intended to limit the present invention. Theinventors compared several approaches to printing 50/50-nm nestedcontact holes using the Prolith™ 7.1 lithography simulation systemavailable from KLA-Tencor Corp. The approaches used include: off-axisquadrupole illumination and attenuating phase-shift mask with optimizedpolarization of the illumination; chromeless alternating phaseshift-masks (CAPSM) in conjunction with special polarization schemes;immersion lithography with extremely high numerical aperture (NA) at157-nm wavelengths; and EUV lithography.

[0074] The inventors found the limits of the off-axis illuminationtechnique can be pushed with the use of radial polarization and how themask bias (or background transmission) can be used to optimize theimage. Resolution limits are further pushed with 2D chromelessalternating PSM combined with the radial polarization. With radialpolarization enhancement according to embodiments of the presentinvention, high-contrast images can be obtained and high-quality contactholes at 100-nm pitch can be printed using negative photo-resist. Radialpolarization according to embodiments of the present invention canfurther enhance image quality in example applications involvingimmersion. The inventors further compared these findings with resultsobtained at an extreme ultraviolet radiation (EUV) wavelength to confirmthat imaging at an EUV wavelength and low NA can also provide excellentconditions to print 100-nm-pitch contact holes.

[0075] A. Introduction

[0076] At present, producing 50-nm contacts with a 100-nm pitchrepresents a challenge for optical lithography. Industry roadmaps forsemiconductor devices require 50 nm contacts to be available by 2008. Toachieve this capability before EUV becomes widely available requires asignificant extension to current optical lithography. Even with high-NAoptics using a 157-nm wavelength, traditional contrast-enhancementtechniques such as off-axis illumination and attenuating PSM are notsufficient to print 100-nm-pitch contact holes of a high enough quality.There is a need to use resolution-enhancing techniques in addition toquadrupole illumination with attenuating PSM for the printing of100-nm-pitch contacts. For reference, FIG. 3A is a zoomed-in diagram ofan example wafer with contact holes. Resist layer 302 is attached towafer surface 304. A lithography system (not shown) exposes resist layer302 to produce contact hole 306. FIG. 3B is an image of the contact holepattern when viewed from above.

[0077] Simulations were used to examine the optimum printing techniquefor grouped contacts according to embodiments of the present invention.The inventors used Prolith™ 7.1 to explore techniques to improvecontact-window printing capabilities. First, the inventors simulatedconventional lithographic conditions for a high numerical aperture157-nm system and establish the minimum pitch that can be printed withadequate image contrast. Next, the inventors progressively modified thetechnique to improve resolution capability. In this way, the presentinventions demonstrates an improvement in results compared with thestarting condition of off-axis illumination and a 6% attenuatedphase-shift mask (PSM) by first optimizing the polarization of theillumination, exploring chromeless phase-shift masks, and then byintroducing wavelength-shortening methods such as immersion lithographyand EUV lithography.

[0078] B. Resolution

[0079] First, the theory of resolution is examined in the context ofprinting contacts. This theory helps explains how one can enhance theresolution of system for contact arrays.

[0080] B.1. Theoretical Resolution Limits

[0081] For a 2D periodic pattern of pitch p_(x) in the x direction andp_(y) in y direction, the mask spectrum is non zero at discrete spatialfrequencies whose x and y components are inversely proportional to thepitch:$\left( {f_{x},f_{y}} \right) = \left( {\frac{n}{p_{x}},\frac{m}{p_{y}}} \right)$

[0082] where n and m are integer numbers (0, +/−1, +/−2, +/−3 etc.)

[0083] Often, it is more convenient to work with normalized spatialfrequencies i.e.:$\left( {{\hat{f}}_{x},{\hat{f}}_{y}} \right) = {\left( {\frac{n \cdot \lambda}{p_{x} \cdot {NA}},\frac{m \cdot \lambda}{p_{y} \cdot {NA}}} \right).}$

[0084] At a minimum, in addition to the zero order, the first threediffraction orders must be captured by the lens to ensure sufficientresolution, i.e., the (0,0), (0,1), (1,0) and (1,1) orders.

[0085] For on-axis illumination, this requirement is equivalent to:

{square root}{square root over ({circumflex over (f)})} _(x) ²+{circumflex over (f)} _(y) ²≦1

[0086] i.e., fitting a square into one quadrant of a unit radius circle(see diffraction pattern in FIG. 4A) $\begin{matrix}{{i.e.\quad p} \geq {\frac{\lambda}{NA} \cdot \sqrt{2}}} & {{if}\quad} & {{p_{x} = {p_{y} = p}},}\end{matrix}$

[0087] and with diagonal off-axis illumination:

{square root}{square root over ({circumflex over (f)})} _(x) ²+{circumflex over (f)} _(y) ²≦2

[0088] i.e., fitting a square into the entire unit radius circle (seediffraction pattern in FIG. 4B), $\begin{matrix}{{i.e.\quad p} \geq {\frac{\lambda}{NA} \cdot \sqrt{2}}} & {{if}\quad} & {{p_{x} = {p_{y} = p}},}\end{matrix}$

[0089] By using this theory, one can establish that at 157-nm wavelengthand 0.93 NA, and for grouped contact holes, the theoretical minimumpitch (in x and y directions) that can be imaged is therefore 240 nmwith on-axis illumination and 120 nm with off-axis illumination. Theinventors ran simulations to further explore the off-axis case.

[0090] B.2. Resolution Capability with Off-Axis Illumination Lithography

[0091] Resolution can be improved by going from conventional on-axisillumination to off-axis illumination. Being able to produce an imageis, by itself, insufficient to meet certain quality criteria to ensuresufficient process latitude in resist. To print nested contact holes,assume that contrast and normalized image log slope (NILS) must begreater than 0.5 and 1.5, respectively. See, Graeupner, P., et al.,“Solutions for printing sub-100 nm contact with ArF,” SPIE 4691:503(2002).

[0092] With these requirements, the inventors determined the smallestpitch at which nested contact holes can be printed in this example.First, example high-quality conditions for printing low-k, groupedcontact holes were considered. These conditions include:

[0093] 0.9/0.1 Quadrupole illumination with diagonal poles (where 0.9 isthe distance of the poles from the center and 0.1 is the pole radius)

[0094] 0.93 NA

[0095] 6% 1:1 Attenuating PSM

[0096] Unpolarized light incident on the reticle

[0097] By gradually decreasing the pitch, the aerial image contrast inthe pitch direction (CO) can stay above 0.5 as long as the pattern pitchis 134 nm (i.e. 67-nm contacts and spaces). This coincides with a NILSof 1.5. The resulting image is shown in FIG. 5.

[0098] The printing resolution pitch of contacts can be improved from240 nm to 134 nm by changing from on-axis illumination to off-axisillumination. Clearly, the smallest resolvable pitch as defined abovewill change somewhat when using a slightly different NA or quadrupole.This definition does not take into account depth of focus (DOF);therefore, the lowest printable pitch is expected to be larger. Thisexample is sufficient to show that fairly unconventional means must beused to push the resolution limit down to 100-nm pitch.

[0099] C. Polarization

[0100] There are references in the literature to “polarization matching”in which the electric field vectors overlap and result in maximuminterference and consequently in maximum image quality. See, Ma, Z., etal., “Impact of illumination coherence and polarization on the imagingof attenuated phase shift masks,” SPIE 4346:1522 (2001). Linearlypolarized illumination has been used to improve image quality for linesof appropriate orientation, but no specific polarization schemes havebeen suggested for contact holes as in embodiments of the presentinvention.

[0101] In the following discussion, the effects of using radially andtangentially polarized light are revealed. These two types ofpolarization enhance image quality for contact holes. Although radialand tangential polarizations are discussed here, one of skill in the artwill recognize with this disclosure that other polarizations, includingcustom polarizations, can also be used to enhance image quality.

[0102] Unless stated otherwise in the following, the NA is 0.93, theillumination is 0.9/0.1 diagonal quadrupole, and the wavelength is 157.6nm.

[0103] C.1. Sintulation Experiment

[0104] The Prolith™ 7.1 simulator used for this work offered a choice ofthree polarization modes, namely, x polarized, y polarized, andunpolarized light. The image for the unpolarized mode was obtained byadding the aerial images of the x polarized and y polarized modes.

[0105] To simulate tangentially or radially polarized light, a simplemanipulation of the mask orientation was carried out using Prolith™ 7.1.An example of the orientation manipulation is illustrated in FIG. 6.Assuming the poles to be sufficiently small, the inventors first rotatedthe pattern by 45°. They then calculated a first image using x-orienteddipole illumination with x or y polarized light (x polarization forradial and y polarization for tangential). Next, a second image wascalculated using y-oriented dipole illumination with x or y polarizedlight (y polarization for tangential and x polarization for radial).Finally, the two images were added to obtain the final image.

[0106] C.2. Effect of Polarization on Image Quality

[0107] For comparison, FIGS. 7A and 7B show images using radial andtangential polarization, respectively, according to the presentinvention. The difference between tangential polarization and radialpolarization is illustrated in FIGS. 10A and 10B. When light isunpolarized, as shown in view 1002, the directions of the polarizationvectors vary randomly. Once the unpolarized light passes throughtangential polarizer 1004, however, the light becomes tangentiallypolarized, as shown in view 1006. Once they are tangentially polarized,the polarization vectors uniformly circle around a central location.

[0108] Radial polarization acts somewhat differently. As illustrated inFIG. 10B, when unpolarized light 1002 passes through radial polarizer1008, the light becomes radially polarized light 1010. Once radiallypolarized, the polarization vectors emanate uniformly from the centrallocation.

[0109] The pitch direction is shown as a diagonal line on FIGS. 7A and7B. The contrast along the pitch direction is very high, i.e., 0.88 withradial polarization, while it is very low (0.19) with tangentialpolarization. At 45 degrees from the pitch direction, contrast is highfor both types of polarization.

[0110] With radially polarized light, the image can be optimized bychanging the contact-hole width (i.e., the mask bias) until the contrastis the same in both the orthogonal and the diagonal directions (withtangential polarization, no such improvement was observed). Acontact-hole width of 85 nm (i.e., 18-nm mask bias) produced uniformcontrast at a pitch of 134 nm (approximately equal diagonal andorthogonal contrasts), in one preferred example.

[0111] Alternatively, it is possible to modify the backgroundtransmittance of the mask to obtain the same effect. The inventors foundthat radially polarized light results in an image quality that issuperior to that produced with unpolarized light, even when out offocus.

[0112] The improved resolution afforded by the optimization ofillumination polarization was examined. The minimum pitch can be reducedto 125 nm, and an optimum contact-hole width found. With radialpolarization, intense side lobes can be observed at small contact-holewidths. The side lobes disappear as the contact-hole width is graduallyincreased from 50 nm to 75 nm. This is followed by a more uniformlydistributed contrast around the contact. The results indicated that theradially polarized case offers more than 30% NILS improvement over theunpolarized light case. FIGS. 8A-8C illustrate the comparison of thethree polarization states according to the present invention. In each ofthese figures, the pitch direction is shown. FIG. 8A is an aerial imageusing unpolarized quadrupole illumination at 75 nm contact width. FIG.8B is an aerial image with 75 nm contact width using tangentiallypolarized quadrupole illumination. Finally, FIG. 8C is an aerial image,again at 75 nm contact width, using radial quadrupole illumination.

[0113] Polarizing the illumination can improve the resolution limit from134 nm down to 125 nm. These figures are based on the use of radiallypolarized light and a minimum contrast requirement of 0.5 (NILSrequirement of 1.5).

[0114] C.3. Polarized Light, Chromeless PSM, Negative Photo-Resist

[0115] In embodiments of the present invention, polarized light(radially or tangentially polarized) is used to illuminate a phase-shiftmask (PSM) and produce an exposure beam. A negative photoresist layer isthen exposed by light in the exposure beam. A chromeless PSM can beused. In one example embodiment, radially polarized light is used inconjunction with chromeless PSMs, Cartesian quadrupole (C-quad)illumination and negative photoresists. A very high image quality isobtained even when printing grouped or nested contact holes in low kapplications. Forbidden pitch is avoided.

[0116] In one example, radially polarized light is used in conjunctionwith chromeless PSMs, Cartesian quadrupole illumination, and negativephoto-resists to push the resolution to k=0.29. The present invention isnot limited to Cartesian quadrupole illumination. Further examplesinclude but are not limited to quasar illumination, illumination havingfour-fold symmetry, or any other illumination approximating quadrupoleillumination. According to simulations performed by the inventors on thePROLITH™ 7.1 system, near perfect contrast of image occur when negativephoto-resist and radially polarized illumination are used.

[0117]FIGS. 9A-9C show the effect of polarization on image quality inthe simulation results obtained by the inventors. The results shown inFIGS. 9A-9C are simulations of 100 nm pitch, chromeless PSM contactholes using a 157 nm wavelength, 0.93 NA and a resist with a refractiveindex of 1.78. FIG. 9A shows a poor contrast image of contact holesobtained in a case using unpolarized light and quadrupole illumination.FIG. 9B shows a poor contrast image of contact holes obtained in a caseusing tangentially polarized light and quadrupole illumination. FIG. 9Cshows a high contrast image of contact holes obtained in a case usingradially polarized light and quadrupole illumination according to anembodiment of the present invention.

[0118] The minimum contrast of the three types of polarization in thisexample is summarized below: Polarization state Minimum contrastUnpolarized 0.67 Tangential 0.44 Radial 1.0

[0119] In addition, this technique has the potential to be used at evenlower k-factor (e.g., where k equals 0.26 with contrast larger than0.75)

[0120] This approach shows no forbidden pitch as demonstrated in FIGS.11A and 11B. FIGS. 11A and 11B are graphs that respectively plot the CD(in nm) and NILS over a range of pitches between 100 and 900 nm,according to the present invention. FIGS. 11A and 11B show that NILSdoes not drop below 2.9 for all simulated pitches (100 to 900 nm in 25 mpitch steps) indicating that all pitches can be printed simultaneouslywith good exposure latitude.

[0121] C.4. Radially Polarized Light, Attenuating Phase-Shift Masks orBinary Masks, and Positive Photoresists

[0122] In further embodiments of the present invention, radiallypolarized light is used to illuminate a phase-shift mask (PSM) andproduce an exposure beam. A positive photoresist layer is then exposedby light in the exposure beam. The mask can be an attenuating PSM orbinary mask.

[0123] An example of an attenuating PSM 1200 is shown in FIG. 12. Forease of explanation, only cell 1202 of attenuating PSM 1200 isdescribed. Central section 1204 of cell 1202 is an area of 100%transmission, meaning that all light of a certain phase passes through.For example, central section 1204 may transmit all light having a phaseof 0°. Outer section 1204 of cell 1202 causes the attenuation, in that alower percent of light of another phase is transmitted. For example, asshown in FIG. 12, outer section 1206 only allows 6% of light at a phaseof 180° to pass through.

[0124] Alternatively, a binary PSM may be used in the present invention.FIG. 13 illustrates an example binary PSM 1300. For ease of explanation,only cell 1302 of binary PSM 1300 is described. Much like centralsection 1204 of attenuating PSM 1200, central section 1304 of binary PSM1300 allows 100% of light through. However, instead of allowing anattenuated degree of light to pass, outer section 1306 prevents alllight from passing through. In other words, outer section 1306 has a 0%transmission rate.

[0125] In one example embodiment, radially polarized light is used inconjunction with attenuating phase-shift masks or binary masks, standarddiagonal quadrupole illumination and positive photoresists. A very highimage quality is obtained even when printing contact holes at variouspitches in low k applications.

[0126] Simulations performed by the inventors on 125 nm pitch contactholes using a 6% attenuating PSM and diagonal quadrupole (0.9/0.1)illumination showed image improvement when radially polarized light wasused. The results from this simulation are shown in FIGS. 14A-14C. Theresults shown in FIG. 14 are also summarized in the following table:Polarization state Minimum Contrast Minimum NILS* Unpolarized 0.64 1.85Tangential 0.58 1.77 Radial 0.69 1.94

[0127] The present invention is not limited to quadrupole illumination.Further examples include, but are not limited to, quasar illumination,illumination having four-fold symmetry, or any other illuminationapproximating quadrupole illumination.

[0128] D. Polarization with Chromeless Alternating PSM

[0129] Further improvements in the resolvable pitch for contacts can bemade by changing the mask from an attenuating phase-shift mask to analternating phase shift mask while retaining the use of polarized light.

[0130] D.1. Chromeless Alternating PSM in Conjunction with RadiallyPolarized Light, 100-nm Pitch Nested Contacts

[0131] The chromeless alternating PSM layouts chosen for this examplestudy are of the checkerboard type in which the phase is alternatedbetween, for example, 0° and 180°. A diagram of a chromeless alternatingPSM 1500 is shown in FIG. 15. The center section of PSM 1500 ishighlighted to showcase the different sections and phases. Sections 1502and 1504 are areas that allow 100% of light to pass through with a phaseof, for example, 0°. Sections 1506 and 1508 are areas that allow 100% oflight to pass through at a phase different from that of sections 1502and 1504. For example, sections 1506 and 1508 may have a phase of 180°.Darkened area 1510 does not allow any light through. Therefore, itstransmission is 0%.

[0132] The repeat pattern for printing 100-nm-pitch contact holes iscomposed of 100 nm transparent squares with alternating phases, asillustrated in FIG. 16. See, mask layouts in Levenson, M. D., et al.,“The vortex mask: making 80 nm contacts with a twist!,” SPIE 4889 (2002)and Grassman, A., et al, “Contact hole production by means of crossingsudden phase shift edges of a single phase mask,” International patentWO 01/22164 A1 (2001). The resulting diffraction pattern for thiscontact array, with on-axis illumination for the chromeless mask, isshown in FIG. 17A.

[0133] With on-axis illumination, no diffraction orders are captured bythe lens because the (0,0), (1,0), and (0,1) orders are extinct. Withoff-axis illumination, according to an embodiment of the presentinvention, the (1,1) family of diffraction orders can be moved into thepupil, as illustrated in FIG. 17B. An image can be obtained with thissetup. With one pole, the image is equivalent to a 1D grating. Bycombining x-poles with y-poles, a 2D image is produced. FIGS. 18A and18B illustrated this 2D image. This image can be enhanced by using theoptimum polarization for the two interfering diffraction orders fromeach pole (in the case shown in FIG. 17B, x-polarized light is theoptimum). This is like radially polarized poles with a Cartesianquadrupole.

[0134] Simulations in air and resist (FIGS. 18A-B) were for NA of 0.93,wavelength 157.6 nm, and the illumination was Cartesian quadrupole (thediagonal quadrupole presents no advantage but can be used). The fourpoles were radially polarized. Both the simulations in air and resistshowed nearly perfect contrast as long as the contact holes were printedin negative photo-resist. NILS (calculated in air only) was very high(larger than 3, as shown in FIG. 18A).

[0135] D.2. Through-Pitch Behavior, Chromeless Contacts with RadiallyPolarized Light

[0136] “Forbidden” pitches in lithography have been described. See,Socha, R., et al., “Forbidden pitches for 130 nm lithography and below,”SPIE 4000:1140 (2000), and Shi, X., et al., “Understanding the forbiddenpitch phenomenon and assist feature placement,” SPIE 4689:985 (2002).For a given illumination angle, the forbidden pitch lies in the locationwhere the field produced by the neighboring features destructivelyinterferes with the field of the main feature. Difficulties areencountered when attempting to print contact holes of a given size atdifferent pitches. See, Graeupner et al. The inventors used commonillumination conditions and a common threshold to mimic simultaneousexposure and to evaluate the extent of any overlapping process windows.

[0137] For this set of simulations, the size of the transparent phasesquares that make up the chromeless mask (see FIG. 16) was graduallyincreased from 100 nm to 1000 nm in 25-nm steps. FIG. 19 shows thevarious images resulting from simulations at best focus for 200-nm,300-nm, 400-nm, 500-nm, 600-nm, and 1000-nm pitches.

[0138] As a result of the present invention, the image of the contactgenerally remains particularly sharp and does not vary significantly insize with pitch. This is because the contact forms at the corners of thephase squares. One threshold necessary to print 50 nm contacts at 100-nmpitch has been calculated and found to be 0.28. At this threshold, sidelobes are seen to develop for pitches of between 400 nm and 500 nm (seeFIG. 19) and, hence, assist features are needed to prevent side lobesfrom printing at these particular pitches. Side lobes do not appear tobe a problem at other pitches and assist features are therefore notrequired.

[0139] NILS and contact width have been calculated at the 0.28 thresholdfor all pitches (see FIGS. 20A and 20B, respectively). The contact widthis the width of the image at the target threshold (0.28, in this case),and NILS is the log slope of the image width at the same threshold.

[0140] As shown in FIG. 20B, the contact width varies almost linearlywith pitch for small pitches (approx. 200 nm); this is the regime inwhich the image is just the sum of orthogonal 1D gratings. Beyond thisrange, more diffracted orders are accommodated in the pupil. Althoughnot shown here, the depth of focus (DOF) shows a change from infinite(with ideal mask, point source, and wave front) to finite, when morediffracted orders contribute to the image.

[0141] As shown in FIG. 20A, the NILS for all pitches underconsideration in this example remains well above 2.5. This indicatesgood exposure latitude for all pitches. Contact width, on the otherhand, varies from 50 nm to 105 nm (worst pitch) and stabilizes at about65 nm. This is quite remarkable and has advantages of the simplicity ofthe mask layouts and the absence of a pitch-dependent pattern andillumination optimization. Compare, Graeupner et al., Socha, R., et al.,and Shi, X., et al.

[0142] D.3. Custom Polarization

[0143] In an embodiment, custom polarized light is used in place ofsimple radial or tangential polarized light. FIG. 21A is a map of anexample custom polarization pattern, where each arrow represents thedirection of the field vectors at a specific section of the light beam.FIG. 21B is another map of an example custom polarization pattern.Unlike radial or tangential polarization, custom polarization patternshave a non-uniform arrangement of polarization vectors. Thesepolarization vectors are shown as arrows in FIGS. 21A and 21B. In anembodiment, custom polarized light, as well as radial and tangentialpolarization, can be produced by a pattern polarizing device, such aspattern polarizing device 104 or 202. The pattern in the patternpolarizing device is predetermined, and the pattern polarization devicecan be changed as necessary to produce the desired polarization. Theillumination configuration, or shape of the illumination light at theillumination source, can be customized as well. The ability to providecustomized illumination, as well as customized polarization andintensity, optimizes printing.

[0144] E. Immersion Lithography

[0145] Another lithographic technique, immersion lithography, can alsobe used for the printing of contacts in the present invention. Inimmersion lithography at least the space between the projection optic,such as projection optic 108, and the wafer, such as wafer 110, isfilled with a liquid. Using immersion lithography, it is possible toextend the pitch-resolution limits from 125 nm down to 100 nm. Tosimulate immersion lithography, the wavelength was scaled by therefractive index of the immersion liquid (e.g., 1.5). The liquid NA thatcould potentially be achieved with a suitable lens design is 1.395. FIG.22 is an image of 50-nm contact holes at a 100-nm pitch simulated withimmersion lithography according to the present invention. The NILS is inexcess of 1.74; this indicates that this is a viable optical lithographytechnique for 50-nm contacts on a 100-nm pitch.

[0146] F. EUV

[0147] EUV was also examined, as it affords very short wavelength and,hence, has a high k factor at 100-nm pitch. An aerial image simulationof the image using typical EUV conditions (0.6 PC, 0.25 NA, and a binarycontact-hole mask with unpolarized light) confirms that EUV can printvery high-quality 50 nm contact images at 100 nm pitch. The result of asimulation using EUV according to the present invention is shown in FIG.23.

[0148] NILS and contrast have both been found to be more than 0.7 and2.5, respectively, over a 0.4-micron DOF; this indicates that EUV could,under the right conditions, provide robust imagery.

[0149] The inventors considered several approaches to the printing of100-nm-pitch nested contact holes and found that 157-nm and high NAallows the printing of 134-nm-pitch contacts with good image quality.Further, radial polarization, in conjunction with a state-of-the-artapproach at 157 nm (attenuated PSM, quadrupole, etc.), can result in anotable improvement compared with results produced using unpolarizedlight. The smallest pitch resolved with this technique is 125 nm. Withradial polarization, C-quadrupole, chromeless alternating PSM andnegative photo-resist, one can obtain nearly perfect contrast images of100-nm-pitch contact holes at 157 nm. The inventors found this approachto be by far the best at 157 nm. Image quality remained almost constantthrough pitch and the inventors did not observe a forbidden pitch. Theinventors discovered that immersion, at 157 nm and in a 1.5 refractiveindex hypothetical fluid, results in high-quality images at 100-nmpitch, while EUV conditions result in very high-quality images for100-nm-pitch contacts.

[0150] The simulation results are summarized in the table below.Wavelength Pitch Method (nm) NA (nm) Contrast NILS Att PSM & unpolarized157.6 0.93 134 0.5 1.55 light Att PSM & radial 157.6 0.93 125 0.51 1.47polarization Chromeless PSM & 157.6 0.93 100 0.99 3.08 radialpolarization Immersion 157.6 1.395 100 0.62 1.74 EUV 13.4 0.25 100 0.995.27

[0151] While various embodiments of the present invention have beendescribed above, it should be understood that they have been presentedby way of example only, and not limitation. It will be apparent topersons skilled in the relevant art that various changes in form anddetail can be made therein without departing from the spirit and scopeof the invention. Thus, the breadth and scope of the present inventionshould not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

What is claimed is:
 1. A method for printing on a wafer, comprising: (a)producing an exposure beam with polarized illumination light, whereinthe illumination light is polarized according to a predeterminedpolarization pattern; (b) illuminating a mask to produce an image in theexposure beam; and (c) exposing a photoresist layer on the wafer withlight in the exposure beam.
 2. The method of claim 1, wherein said step(a) further comprises producing polarized illumination light accordingto a radial polarization pattern.
 3. The method of claim 1, wherein saidstep (a) further comprises producing polarized illumination lightaccording to a tangential polarization pattern.
 4. The method of claim1, wherein said step (a) further comprises producing polarizedillumination light according to a custom polarization pattern.
 5. Themethod of claim 1, wherein said step (a) further comprises producingpolarized quadrupole illumination.
 6. The method of claim 1, furthercomprising before said step (a): emitting pre-polarized light to produceillumination light.
 7. The method of claim 1, wherein said step (b)comprises illuminating a mask to produce an image that includes contactholes.
 8. The method of claim 1, wherein said step (c) occurs in aliquid.
 9. The method of claim 1, wherein the mask is at least one ofthe group consisting of: chromeless phase-shift mask, attenuatingphase-shift mask, and alternating phase-shift mask.
 10. The method ofclaim 1, wherein the mask is a binary mask.
 11. A method of printing ona wafer, comprising: (a) producing an exposure beam with polarizedillumination light, wherein the illumination light is polarizedaccording to a predetermined polarization pattern; (b) illuminating achromeless phase-shift mask to produce an image in the exposure beam;and (c) exposing a negative photoresist layer on the wafer with light inthe exposure beam.
 12. A method of printing on a wafer, comprising: (a)producing an exposure beam with polarized illumination light, whereinthe illumination light is polarized according to a predeterminedpolarization pattern; (b) illuminating an attenuating phase-shift maskto produce an image in the exposure beam; and (c) exposing a positivephotoresist layer on the wafer with light in the exposure beam.
 13. Amethod of printing on a wafer, comprising: (a) producing an exposurebeam with polarized illumination light, wherein the illumination lightis polarized according to a predetermined polarization pattern; (b)illuminating a binary mask to produce an image in the exposure beam; and(c) exposing a positive photoresist layer on the wafer with light in theexposure beam.
 14. A method of printing on a wafer, comprising: (a)illuminating a phase-shift mask with pre-polarized light; (b) shapingsaid pre-polarized light to produce an exposure beam, wherein thepre-polarized light is shaped according to a predetermined polarizationpattern and intensity pattern; and (c) exposing a photoresist layer onthe wafer with the exposure beam.
 15. A lithography system, comprising:(a) an illumination source that emits illumination light along anoptical path; (b) a pattern polarizing device that converts illuminationlight from the illumination source into an exposure beam with apredetermined polarization pattern; (c) a mask that produces an image inthe exposure beam; (d) a projection optic that relays the exposure beamfor printing on a wafer.
 16. The lithography system of claim 15, whereinsaid illumination light is pre-polarized illumination light, and whereinsaid pattern polarizing device is a wave plate.
 17. The lithographysystem of claim 15, wherein said illumination light is pre-polarizedillumination light, and wherein said pattern polarizing device is apolarizer.
 18. The lithography system of claim 15, wherein saidillumination light is unpolarized illumination light, and wherein saidpattern polarizing device is a polarizer.
 19. The system of claim 15,further comprising: (e) a wafer exposed by the exposure beam.
 20. Thelithography system of claim 19, further comprising a liquid filling aspace between said projection optic and said wafer.
 21. The lithographysystem of claim 15, wherein said pattern polarizing device is includedin the projection optic.
 22. The lithography system of claim 15, whereinsaid predetermined polarization pattern is a radial polarizationpattern.
 23. The lithography system of claim 15, wherein saidpredetermined polarization pattern is a tangential polarization pattern.24. The lithography system of claim 15, wherein said predeterminedpolarization pattern is a custom polarization pattern.
 25. Thelithography system of claim 15, wherein said mask is one of the groupconsisting of: a chromeless phase-shift mask, an attenuating phase-shiftmask, a binary mask, and an alternating phase-shift mask.
 26. Thelithography system of claim 15, wherein said image includes contactholes for a wafer.
 27. A method of producing contact holes on a wafer,comprising: (a) producing a polarized illumination beam; (b)illuminating a mask with the polarized illumination beam to create anexposure beam, wherein said mask produces a contact hole image in theexposure beam; and (c) exposing a wafer with the exposure beam.
 28. Themethod of claim 27, wherein said step (b) further comprises illuminatinga phase-shift mask.
 29. The method of claim 27, wherein said step (a)further comprises producing a radially polarized illumination beam. 30.The method of claim 27, wherein said step (a) further comprisesproducing a tangentially polarized illumination beam.
 31. The method ofclaim 27, wherein said step (a) further comprises producing a custompolarized illumination beam.